Gray level mask

ABSTRACT

A gray level mask suitable for photolithography is constructed of a transparent glass substrate which supports plural levels of materials having different optical transmissivities. In the case of a mask employing only two of these levels, one level may be constructed of a glass made partially transmissive by substitution of silver ions in place of metal ions of alkali metal silicates employed in the construction of the glass. The second layer may be made opaque by construction of the layer of a metal such as chromium. The mask is fabricated with the aid of a photoresist structure which is etched in specific regions by photolithographic masking to enable selective etching of exposed regions of the level of materials of differing optical transmissivities. Various etches are employed for selective etching of the photoresist, the metal of one of the layers, and the glass of the other of the layers. The etches include plasma etch with chloride ions to attack the chromium of the opaque layer, compounds of fluorine to attack the glass layer, and reactive ion etching with oxygen to attack the photoresist structure. Also, developer is employed for etching on hardened regions of resist in the photoresist structure.

This is a divisional of copending application Ser. No. 07/605,606 filedon Oct. 30, 1990 now U.S. Pat. No. 5,213,916.

BACKGROUND OF THE INVENTION

This invention relates to the formation of masks useful inphotolithography for the fabrication of semiconductor devices, and moreparticularly, to a construction of masks having regions of material ofdifferent optical opacity, the regions being arranged typically as asuccession of layers.

In the use of photolithography for the construction of semiconductordevices as well as other devices such as masks and reticles, theconstruction process involves numerous steps which include the exposureof photoresist through a mask to delineate specifically shaped areaswhich are to be etched. There are situations in which a plurality ofexposure steps are to be performed sequentially with a plurality ofmasks to accomplish differing amounts of exposure of the various areasto be etched.

The manufacturing process can be simplified by reduction of the numberof exposure steps by use of a gray level mask. A gray level mask allowsa defining of two or more conventional mask levels in a single exposurestep. This technique lends itself to process clustering with itspotential for low defect density, and also facilitates manufacture byreducing the number of masking levels required. The technique alsoreduces tolerances between gray level patterns. By way of example, animportant use of gray level mask technology is for recessed multilevelwiring applications where via and wiring patterns may be produced withone exposure.

A problem arises with presently available gray level masks in that themasks are difficult to fabricate and, furthermore, produce inadequateimage quality in many applications. By way of example in the fabricationof a gray level mask, it has been the practice to form the mask by anarray of spaced-apart chromium islands in the nature of a half-tonescreen wherein each chromium island is opaque to ultraviolet radiationwhile spaces between the islands allow passage of the radiation. Thehalf-tone screen is constructed by use of electron-bream lithography soas to produce spaces between the islands wherein the spaces havedimensions smaller than a wavelength of the optical radiation. Theislands may also have dimensions smaller than the wavelength of theoptical radiation. As a result, there is a significant attenuation ofthe optical radiation transmitted through the mask. The resultingtransmissivity of the mask is significantly more than that of a totallyopaque mask region and significantly less than that of a totallytransparent mask region. Thus, the resulting mask is a gray level mask,but a mask which produces a lower quality image than is desired. Theamount of transmissivity is defined by the dimensions of the chromiumislands and the spaces.

A further disadvantage of the foregoing gray level mask is the fact thatimages formed in the gray areas have sloped sidewalls which areunacceptable for use in producing semiconductor products requiring thehigher resolution for condensed packaging of circuit elements as arebeing contemplated for the near future. With respect to other techniqueswhich have been employed in the fabrication of gray level masks, therehas been the disadvantage that the other fabrication methods requireprecise electron-beam dose control to make optical masks orelectron-beam proximity correction, and produce gray layers of specificopacity.

SUMMARY OF THE INVENTION

The aforementioned problem is overcome and other advantages are providedby a construction of gray level mask which, in accordance with theinvention, employs materials having differing opacity to visible light.The term "light" as used herein is understood to include those portionsof the spectrum, such as visible and ultraviolet light which are used inphotolithographic processes for construction of electric circuits insemiconductor chips. Furthermore, the materials are selected incombination with specific etching agents which differentially attack aspecific one of the materials without attacking the other materials.This permits the mask to be produced by conventional means ofphotolithography wherein regions of the various materials can bedeposited in selected regions of the mask, as well as in a succession oflayers superposed on each other.

The invention is advantageous because the fabrication of the gray levelmask can be accomplished by use of optical, x-ray, or electron beamlithography techniques to produce a desired pattern of regions Ofvarious opacity upon the mask. It is understood that the principles ofthe invention can be employed in the fabrication of other structuressimilar to a mask, such as a reticle.

BRIEF DESCRIPTION OF THE DRAWING

The aforementioned aspects and other features of the invention areexplained in the following description, taken in connection with theaccompanying drawing wherein:

FIGS. 1-14 show a succession of steps in the fabrication of a mask inaccordance with the invention, the steps being indicateddiagrammatically;

FIG. 15 shows, diagrammatically, a photolithographic system employingthe mask (as a reticle) of the invention;

FIGS. 16-18 disclose in stylized perspective view an adjustment ofoptical transmissivity in a glass film layer of a completed mask by useof a laser heating beam and an electron writing beam for reconfiguring arelatively opaque mark on the glass film; and

FIGS. 19-31 show a succession of steps in the fabrication of a mask inaccordance with an alternative fabrication process of the invention, thesteps being indicated diagrammatically.

DETAILED DESCRIPTION

The procedure for fabrication of a gray level mask 20, shown incompleted form in FIGS. 14 and 15, is accomplished in accordance withthe invention, as shown in FIGS. 1-14. The procedure begins with asubstrate 22 (FIG. 1) upon which is deposited a succession of threelayers of material, namely, a first layer 24A, a second layer 24B, and athird layer 24C to provide a structure 26 (FIG. 4). A further layer 24Dis subsequently deposited to provide a further structure 28 (FIG. 7). Inthe structure 28, the substrate 22 supports a set of four layers 24. Thethird layer 24C has an opening 30 filled with material of the fourthlayer 24D. In the ensuing description, the legend 24 may be employed toidentify the layers generally, with the suffixes A-D being employed toidentify specific ones of the layers. The structures 26 and 28 areemployed with various lithographic steps (FIGS. 5 and 8) and etchingsteps (FIGS. 6 and 9-14) to produce the completed mask 20.

The fabrication procedure is described now in greater detail. Withreference to FIG. 1, the substrate 22 is constructed, typically, of atransparent material, such as glass, which is suitable for use inlithographic procedures employed in various manufacturing processes suchas the fabrication of semiconductor devices, masks and reticles, by wayof example, wherein the mask 20 may be used to advantage. In FIG. 2, thefirst layer 24A is deposited upon the top surface of the substrate 22.

The first layer 24A is fabricated of a material which has greateropacity and reduced transmissivity to the propagation of radiation suchas light than does the material of the substrate 22. A similar maskstructure built with different materials may be suitable forx-radiation. Preferably, the material of the first layer 24A comprises aglass which has been doped with an optical transmission inhibitor suchas a coloring agent or a darkening agent which inhibits the propagationof light so as to reduce the optical transmissivity of the first layer24A to a fraction of the transmissivity of the substrate 22, while stillpermitting a significant amount of light to propagate through the firstlayer 24A. By way of example, the optical transmissivity of the secondlayer 24A may have a value lying in a range of 1/8-7/8 the opticaltransmissivity of the substrate 22.

It has been found useful, in accordance with a preferred embodiment ofthe invention, to fabricate the first layer 24A of a glass materialconstructed in accordance with the teaching of three U.S. Pat. Nos. toWu, namely, 4,894,303; 4,670,366; and 4,567,104. In the Wu patents,there is an exchange of silver ion with the metal ions of alkali metalsilicates and oxides employed in the glass. Characteristics of the baseglass composition, particularly the light transmissivity, can be variedby photoreduction of silver ions. A useful feature of the glass is thefact that the amount of light transmissivity can be adjusted by exposureto an electron beam for darkening the glass, and by exposure to heatabove approximately 200 degrees Celsius for recovering transparency. Thefirst layer 24A may be referred to as a filter glass. The capacity fordarkening by use of the electron-beam radiation, and the capacity forregaining transparency by heating with a laser are indicated in simplefashion diagrammatically in FIG. 2, and are explained in further detailsubsequently with reference to FIGS. 16-18.

In FIG. 3, the second layer 24B is deposited on the top surface of thefirst layer 24A. The deposition of both of the layers 24A and 24B areaccomplished in accordance with well-known practice in the manufactureof multiple-layered semiconductor devices. The optical transmissivity ofthe material of the second layer 24B is less than that of the firstlayer 24A. By way of example, the optical transmissivity of the secondlayer 24B has a value lying in a range of 1/2 the optical transmissivityof the substrate 22 to zero transmissivity, a zero transmissivity beingobtained by making the second layer 24B opaque. By way of example in theconstruction of the mask 20, the preferred embodiment of the mask 20 hasbeen fabricated by constructing the second layer 24B of an opaquematerial, namely, a layer of metal, preferably chromium. The respectivethicknesses of the layers 24A and 24B are selected as a matter ofconvenience in the process of manufacturing the mask 20, a suitablerange of thickness for each of the layers 24A and 24B being the range ofapproximately 0.05 micron to 1/2 millimeter.

In FIG. 4, the third layer 24C is deposited upon the top surface of thesecond layer 24B. The layer 24C comprises resist material, such asphotoresist which is deposited by use of conventional techniqueswell-known in the manufacture of semiconductor devices. The layer 24Cmay be referred to as Resist 1. The step in FIG. 4 is the first of asequence of steps for providing a resist structure comprising aplurality of layers of resist material. A second resist layer of theresist structure is shown as a fourth layer 24D in FIG. 7. A feature inthe construction of the resist structure is the configuring of the thirdlayer 24C to provide the opening 30 prior to deposit of the fourth layer24D.

FIGS. 5 and 6 show, respectively, photolithography and etching stepsemployed in configuring the layer 24C to provide the opening 30. FIG. 6also shows a step of hardening the photoresist material of the thirdlayer 24C subsequent to the etching process.

In FIG. 5, the photolithography employs a mask 32 and a source ofradiation such as light or electron-beam or x-radiation represented by alamp 34, the mask 32 having an opaque region 36 and a transparent, oropen, region 38. The lamp 34 and the mask 32 are arranged above thestructure 26 with the mask 32 being disposed between the lamp 34 and thethird layer 24C. Light from the lamp 34 is directed toward the mask 32with rays of the light propagating through the transparent region 38 toimpinge upon a layer 24C of the photoresist. The illuminated region ofthe photoresist of the layer 24C is activated by the light to respond toa subsequent step of etching (FIG. 6). In accordance with well-knownpractices in the manufacture of semiconductor devices, etching may bedone with a developer such as potassium hydroxide which etches away theilluminated portion of the photoresist material to form an opening 30 inthe layer 24C. The etchant is provided, by way of example, by an etchantsource 40 depicted as a device for delivering etchant as by spraying theetchant on the layer 24C. Subsequent to the etching step, a source 42 ofultraviolet radiation is employed to radiate the layer 24C so as toharden the material of the layer 24C, the term hardening being used toindicate that the resist material is no longer sensitive to illuminationby light of the lamp 34. The hardening step may or may not be requireddepending on the formulation of the photoresist and subsequent etches.The foregoing description relates to a positive photoresist, it beingunderstood that the practice of the invention includes a mask made by anegative resist.

In FIG. 7, fabrication of the resist structure continues with depositionof the fourth layer 24D, this being a layer of photoresist as has beennoted above. The fourth layer 24D may be referred to as Resist 2. Thedepth, x, of the fourth layer 24D is shown greater than the depth, y, ofthe third layer 24C for purposes of illustrating the invention, it beingunderstood, also, that x may be equal to or less than y if desired.

In FIG. 8, the lamp 34 and a mask 44 are arranged above the structure 28with the mask 44 disposed between the lamp 34 and the fourth layer 24D.The mask 44 is aligned to the structure 28, and has an opaque region 46and a transparent, or open, region 48. The lamp 34 directs light towardsa mask 44 with rays of the light propagating through the transparentregion 48. The rays of light from the lamp 34 penetrate both the fourthlayer 24D and the third layer 24C. Resist material of the fourth layer24D is activated by the light to be responsive to an etchant of asubsequent etchant step depicted in FIG. 9. However, the resist materialof the third layer 24C has been hardened (FIG. 6) and, as noted above,has been rendered insensitive to illumination by the light of the lamp34. Therefore, in the subsequent etching step of FIG. 9, only the resistmaterial of the layer 24D will be affected by the etchant, and theconfiguration of the layer 24C including the configuration of theopening 30 will remain unchanged in the subsequent etching step of FIG.9.

In FIG. 9, the etchant source 40 is employed again to remove photoresistfrom the region illuminated in FIG. 8. By way of example in the use ofthe photolithographic step of FIG. 8 and the etching step of FIG. 9, aboundary 50 (FIG. 8) of the illuminated region, and a resulting edge 50A(FIG. 9) of a recess 52 produced by the etching of FIG. 9 intersect theopening 30 (FIG. 8) resulting in a smaller opening 54 (FIG. 9) whichextends from the recess 52.

In FIGS. 10 and 11, the opening 54 is extended in length by etching awaya part of the second layer 24B (FIG. 10) and a corresponding part of thefirst layer 24A (FIG. 11). In accordance with a feature of theinvention, in FIG. 10, the etching of the material of the second layer24B is accomplished by use of an etchant which selectively etches thematerial of the layer 24B without having any significant etching effecton the materials of the other layers 24. Thus, in FIG. 10, the chromiummay be etched by use of liquid ceric ammonium nitrate in nitric acid, orby use of a dry ion plasma etch employing chloride ions in the plasma.For example, the plasma etch may be accomplished in a vacuum employingan electric field, with the vacuum chamber employing argon, and carbontetrachloride as a source 56 of these ions.

Subsequently, in FIG. 11, the source 58 provides an etchant suitable forthe selective etching of the material of the first layer 24A, theetchant of FIG. 11 having essentially no effect upon the material ofother ones of the layers 24. In a preferred embodiment of the invention,the preferred etchant in the etching step of FIG. 11 is a dry plasmaetch, a reactive ion etch (RIE) of CHF₃ +O₂ which is well suited forproducing a narrow trough, or a narrow diameter hole such as theextension of the opening 54. By way of example in the use of thecompleted mask, the hole may be used to form a via connection with aconductor shape formed in the trough in a subsequent metalizationprocedure. Alternatively, a wet etch of buffered hydrofluoric acid maybe employed. Other etches which may be employed for etching the materialof the first layer 24A do not have as great a selectivity in the etchingbecause they tend to attack also the materials of the layers 24, andmake dimensional control more difficult. Such etches are nitrogentrifluoride plus argon, and also carbon tetrafluoride plus oxygen.

The foregoing selection of materials to be used in the various layers24, as well as etches which can selectively etch the materials ofindividual ones of the layers 24 demonstrates an important aspect of theinvention which allows the mask 20 to be constructed by use ofphotolithographic and etching steps. As shown in the foregoing steps,the invention has provided also for the extension of the opening 54 by adepression 60 into the first two layers 24A and 24B. It is also notedthat also, by locating the boundary 50 (FIG. 8) of the light beam withinthe opening 30, and the resultant production of the edge 50A (FIG. 9),it has been possible by back-filling the opening 30 (FIG. 7) withmaterial of the fourth layer 24D to create the opening 54 and thedepression 60 with a cross-sectional dimension which is smaller thanthat of the opening 30.

In FIG. 12, a source 62 of etchant is employed to remove the portion ofthe third layer 24C lying between the boundary 50 and a boundary 64(FIG. 8) of the light beam defined by the transparent region 48 of themask 44. In the etching step of FIG. 9, an edge 64A of the recess 52 isproduced at the location of the boundary 64 of the light beam. The edge64A is extended through the third layer 24C up to the top surface of thesecond layer 24B by the etching process of FIG. 12. The etching in FIG.12 is accomplished by RIE with oxygen in an optional environment ofargon. It is noted that the etching of FIG. 12 attacks both theunhardened layer 24D and the hardened layer 24C without attacking thefirst two layers 24A and 24B. This is in accordance with the above notedaspect of the invention in which etches are selected to provide forselected etching in accordance with the selection of materials of thevarious layers 24.

It is noted that the etchant of FIG. 12 attacks the resist in both ofthe layers 24D and 24C. However, since the layer 24D was prepared with athickness greater than that of the layer 24C, some of the layer 24Dremains in the structure of FIG. 12 to provide increased protection ofthe layer 24B during the etching step of FIG. 13. If the layer 24D wereprepared with a thickness less than that of the layer 24C, then, in FIG.12, none of the material in the horizontal portions of the layer 24Dwould appear, and the layer 24C would appear reduced in thickness.

FIGS. 12-14 show the final steps in the fabrication of the mask 20. Theetching of FIG. 12 has exposed a portion 66 of the second layer 24Blying between the depression 60 and the edge 64A. Exposure of theportion 66 in FIG. 12 prepares the portion 66 for a further etchingshown in FIG. 13. FIG. 13, the etching step is performed by use of thesource 56, previously disclosed in FIG. 10, for etching the chromium ofthe second layer 24B. This produces the structure shown in FIG. 13. Theetching away of the portion 66 of the second layer 24B exposes acorresponding portion 68 (FIG. 13) of the first layer 24 A. The portion68 extends from the depression 60 to the edge 64A. Finally, in FIG. 14,there is a further etching step utilizing the source 62, previouslydescribed in reference to FIG. 12, in which the remaining portions ofthe material in each of the layers 24C and 24D are removed, by example,with O2 RIE.

FIG. 14 shows the resulting mask 20 which is constructed to provide fordiffering transmissivities of light. The regions of the mask 20 havingportions of the second layer 24B are opaque to light. The exposedportion 68 of the first layer 24A is partially transmissive to light. Inthe depression 60, both of the layers 24A and 24B are absent so as topermit full transmission of light through the transparent substrate 22.Thereby, the mask 20 attains the gray scale function by providing fulltransmission of light at the site of the depression 60, partialtransmission of light at the exposed portion 68 of the first layer 24A,and zero transmission of light at the remaining portions 70 and 72 ofthe opaque layer 24B.

FIG. 15 shows diagrammatically a lithographic system employing the mask20 in conjunction with a lamp and a table for holding a workpiece. Thetable positions the workpiece in front of the lamp, and the mask 20 ispositioned between the lamp and the workpiece for illuminating theworkpiece with a pattern of light established by the mask 20. Thepattern of light includes shaded regions produced by the portions 70 and72, a strongly illuminated region at the depression 60 and a weaklyilluminated region at the portion 68. Thereby, the mask 20 can beemployed in a lithographic process for providing differing levels ofillumination.

It is noted that, in accordance with the practice of the invention,additional layers of partially transmissive material may be employed inthe construction of a mask, such as the mask 20, to provide multiplelayers of gray level illumination.

In accordance with a further feature of the invention, it is noted thatthe glass filter of the first layer 24A has an opacity produced by thepresence of the substituted silver ions (as described in theaforementioned patents of Wu). As has been described above, the opacitycan be increased by exposing the first layer 24A to radiation of anelectron beam, and can be reduced by a heating of the first layer 24A byexposure to a laser beam. An increase in the opacity produces a decreasein the optical transmissivity of the glass of the layer 24A, andvice-versa. Advantage may be taken of this characteristic of the glassfilter in the fabrication of the mask, as is disclosed in FIG. 2wherein, subsequent to the step of depositing the first layer 24A, butprior to the step of depositing the second layer 24B, the layer 24A maybe radiated with an electron beam for reacting with the silver toincrease the opacity, or by radiating with a laser to heat the layer 24Ato reverse the reaction of the silver so as to regain the optical, orlight, transmissivity. These additional fabrication steps, as indicatedin simplified form in FIG. 2, provide greater versatility in thefabrication of the mask 20 so as to allow customizing of the mask 20 foruse in specific manufacturing processes, such as the manufacture of aspecific semiconductor device.

With reference also to FIGS. 16-18, if desired, the electron beam can bedirected to specific regions of the first layer 24A by use of awell-known mask or beam-deflection electronics to produce differinglevels of opacity at different locations of the glass filter of thefirst layer 24A. FIG. 16 shows a situation wherein the layer 24A of acompleted mask 20 is provided with a relatively opaque mark locatedwithin a surrounding region of greater opacity. By way of example, themark is portrayed as an elongated arcuate mark having a width greaterthan is desired for a specific manufacturing application in which themask 20 is to be employed. It is desired to reduce the width of themark. This is accomplished by irradiating the portion 68 of the glassfilter layer 24A with a laser beam (FIG. 17) to heat the glass material,and reduce the opacity of the mark to that of the surrounding region.Then, as shown in FIG. 18, electron-beam apparatus provides a beam,considerably narrower than the laser beam and having a shorterwavelength than the laser beam, for writing a new mark of desired widthupon the exposed portion 68 of the layer 24A. The electron beam reactswith the glass material to decrease light transmissivity in thelocations illuminated by the electron beam and, thereby generate themark with the desired reduction in width.

FIGS. 19-31 show a sequence of steps of a process for the manufacture ofthe mask 20, this process being an alternative to the process disclosedin FIGS. 1-14. There are similarities between the two processes. FIGS.27-30 show three layers 24A, 24B and 24C which are composed of the samematerials and have the same thicknesses as do the layers 24A, 24B and24C of FIGS. 4-13. Also, in both processes, the foregoing layers aresupported upon the substrate 22. The etchant sources 40, 56, and 58 areemployed also in the alternate process of FIGS. 19-31, and function inthe same manner as has been described with reference to the process ofFIGS. 1-14. In addition, the lamp 34 and the masks 32 and 44 areemployed in the alternate process of FIGS. 19-31, and function in thesame manner as has been described with reference to the process of FIGS.1-14.

The alternate process for fabrication of the mask 20 begins withpreparation of the substrate 22 in FIG. 19. Thereupon, the glass layer24A is deposited upon the top surface of the substrate 22 in FIG. 20,this being followed by a deposition of the resist layer 24C upon the topsurface of the glass layer 24A in FIG. 21. In FIG. 22, the mask 32 andthe lamp 34 are employed to illuminate the resist layer 24C withradiation to expose predetermined regions of the resist layer 24C incorrespondence with the image of the mask 32, one such exposed regionbeing shown in FIG. 22 beneath a transparent region 38 of the mask 32.

The process continues with three etching steps disclosed in FIGS. 23-25.In FIG. 23, the etchant of the source 40 removes the portion of theresist layer 24C which has been exposed to the radiation in FIG. 22, theremoval of the portion of the layer 24C producing the opening 30 andconfiguring the layer 24C in the form of a mask to be used in thesubsequent etching steps of FIGS., 24 and 25. It is noted that, in FIG.23, the etchant attacks only the resist layer 24C, and leaves the glasslayer 24A intact. In FIG. 24, the etchant of the source 58preferentially attacks the exposed portion of the glass layer 24A,beneath the opening 30, and leaves the resist layer 24C intact. Theetchant of the source 58 removes the material of the exposed region ofthe glass layer 24A down to the top surface of the substrate 22 to formthe depression 60 in the glass layer 24A. Upon completion of thedepression 60, the remainder of the resist layer 24C is stripped away byetchant of the source 40, as shown in FIG. 25.

In FIG. 26, a chromium layer 24B is deposited conformably upon the topsurface of the glass layer 24A and upon the exposed portion of thesubstrate 22 at the bottom of the depression 60. This is followed, inFIG. 27, by a deposition of a resist layer 24C upon the top surface ofthe chromium layer 24B. In FIG. 28, the lamp 34 and the mask 44 areemployed to expose predetermined regions of the resist layer 24C withradiation of the lamp 34. The radiation propagates through transparentregions of the mask 44, one transparent region 48 being shown in FIG.28. The exposure of the predetermined regions of the resist layer 24C bythe radiation in FIG. 28 prepares the resist layer 24C for subsequentetching steps of FIGS. 29 and 30.

In FIG. 29, etchant of the source 40 attacks the regions of the resistlayer 24C which have been exposed to the radiation in the step of FIG.28, the etchant of the source 40 removing material of the resist layer24C in the exposed regions while leaving the chromium layer 24B intact.The removal of material at the predetermined regions of the resist layer24C leaves openings, one such opening 74 being shown in FIG. 29, whichconfigure the resist layer 24C in the form of a mask suitable for theetching step of FIG. 30. In FIG. 30, etchant of the source 56 propagatesthrough the opening 74 to etch preferentially the chromium of the layer24B while leaving material of the resist layer 24C and material of theglass layer 24E and material of the substrate 22 intact. In particular,it is noted that the etching step of FIG. 30 removes chromium from thevertical and horizontal surfaces of the depression 60 of the glass layer24A, and also exposes the portion 68 of the top surface of the glasslayer 24A to produce the configuration of a step as shown in FIG. 30.Finally, at FIG. 31, the remaining portion of the resist layer 24C isstripped away to produce the completed mask 20. The stripping of theresist layer 24C in FIG. 31 is accomplished in the same fashion as hasbeen disclosed in FIG. 25 by use of the etchant of the source 40.Thereby, the alternative process of FIGS. 19-31 has produced the mask 22with the same configuration as has been fabricated by the process ofFIGS. 1-14.

It is to be understood that the above described embodiments of theinvention are illustrative only, and that modifications thereof mayoccur to those skilled in the art. Accordingly, this invention is not tobe regarded as limited to the embodiments disclosed herein, but is to belimited only as defined by the appended claims.

What is claimed is:
 1. A gray level mask comprising:a substrate oflight-transmissive material; a first layer of material having a firsttransmissivity to light less than a light transmissivity of saidsubstrate, exposure of said material of said first layer to radiationhaving a wavelength different from a wavelength of said light altering alight transmissivity of said material of said first layer, saidradiation being drawn from a class of radiation including electron beamradiation and x-radiation; a second layer of material having a secondlight transmissivity less than the light transmissivity of saidsubstrate but greater than or equal to zero transmissivity; wherein saidfirst and said second layers are supported by said substrate with saidfirst layer lying between said substrate and said second layer; saidfirst layer partially laps said substrate to leave an exposed part ofsaid substrate to permit propagation of light at a relatively high levelof intensity via said exposed part of said substrate; and said secondlayer partially laps said first layer to expose a part of said firstlayer, said second layer retaining exposure of said exposed part of saidsubstrate to permit direct illumination of the exposed parts of saidsubstrate and said first layer by said radiation, the exposed part ofsaid first layer permitting propagation of light at a moderate level ofintensity, and said second layer providing for little or no lighttransmission.
 2. A mask according to claim 1 wherein said mask isoperative with radiation incident to a plane of the mask substrate; andwherein said first layer comprises a glass having an opticaltransmission inhibitor.
 3. A mask according to claim 2 wherein saidsecond layer is opaque; and said first layer partially overlies saidsubstrate, and said second layer partially overlies said first layer. 4.A mask according to claim 2 wherein said second layer is metallic, andsaid radiation includes a laser beam for heating the glass of said firstlayer to increase light transmissivity of the glass of the first layer,a light transmissivity of the glass of the first layer being reduced inresponse to illumination by electron beam radiation.
 5. A gray levelmask comprising:a substrate of a material having a predetermined valueof transmissivity to light; a first layer of material disposed on saidsubstrate and having a first value of transmissivity to the light,exposure of said material of said first layer to radiation having awavelength different from a wavelength of said light altering a lighttransmissivity of said material of said first layer, said radiationbeing drawn from a class of radiation including electron beam radiationand x-radiation; a second layer of material disposed on said first layerand having a second value of transmissivity to the light; wherein saidfirst layer partially laps said substrate to leave an exposed part ofsaid substrate to permit propagation of light at a relatively high levelof intensity via said exposed part of said substrate; and said secondlayer partially laps said first layer to expose a part of said firstlayer, said second layer retaining exposure of said exposed part of saidsubstrate to permit direct illumination of the exposed parts of saidsubstrate and said first layer by said radiation, the exposed part ofsaid first layer permitting propagation of light at a moderate level ofintensity, and said second layer providing for little or no lighttransmission.